1. Field of the Invention
The present invention relates to a variable-speed dynamotor-for use as a motor-generator in an electrical power system, and more particularly to a variable-speed dynamotor including a stator which has a first set of windings that provide a number of poles for rotating a rotor and a second set of windings that provide a number of poles different from the number of poles provided by the first set of windings, for generating radial forces acting on the rotor to control the radial position of the rotor, suppress vibrations of the rotor, adjust rotational balancing of the rotor, or control the radial damping of the rotor.
2. Description of the Related Art
Variable-speed dynamotors or motor-generators for use in electric power systems have a rotor which can be rotated at a variable speed. The variable-speed dynamotor has already been put to use, and can increase the stability of the electrical power system in which it is incorporated, with the inertial energy of the rotor by varying the rotational speed of the rotor. The variable-speed dynamotor has been reported in various documents including (1) "395MVA variable-speed system for pumped power generation by Keiji Saito, IEEJ (Institute of Electrical Engineers of Japan) Transaction D. Vol. 113, No. 2, p. 267, 1993, and (2) "Variable-speed pumped generation system"by Shaku Fujimoto, Power Electronics Research Society Journal Vol. 16, pp. 16-26.
FIG. 1 of the accompanying drawings shows a conventional variable-speed dynamotor of general configuration. The variable-speed dynamotor shown in FIG. 1 has a stator 11 and a rotor 12 which are of the same structure as those of a wound-rotor induction machine. Specifically, the stator 11 has three-phase windings connected to power system terminals 10, and the rotor 12 has three-phase windings connected through slip rings to a semiconductor power converter 15. The semiconductor power converter 15 supplies variable-frequency currents to the rotor 12 depending on the rotational speed of the rotor 12, the frequency of the bus terminals of the power system, and so on. The semiconductor power converter 15 is connected to the power system terminals 10 for exchanging electrical energy with the power system terminals 10.
The rotational speed of the variable-speed dynamotor shown in FIG. 1 is variable in a very small range of about 10%. The variable-speed range cannot easily be expanded because of the limited mechanical strength of the rotor 12 and also the mechanical resonance of the rotor 12.
The mechanical resonance of the rotor 12 may be removed by improving the mechanical design of the rotor 12. However, it is a simpler approach to actively vary the damping capability and stiffness of the rotor 12 with magnetic bearings.
Magnetic bearings are disclosed in detail in a document (3) "Magnetic bearing and its related technology I. Controlled magnetic bearing and its applications" by Fumio Matsumura, IEEJ (Institute of Electrical Engineers of Japan) Transaction D. Vol. 114, pp. 1200-1207, 1994, for example.
If magnetic bearings are incorporated in a variable-speed dynamotor, then the axial length of the rotor is increased, making the mechanical system of the variable-speed dynamotor complex, and lowering the critical speed of the variable-speed dynamotor. Therefore, the controllability of the variable-speed dynamotor is reduced by the use of the magnetic bearings. It is desirable to generate radial forces on the rotor supplementally or actively without modifying the mechanical system, e.g., the length of the rotor, of the variable-speed dynamotor.
FIG. 2 of the accompanying drawings illustrates an ultra-high-speed rotary machine system which comprises an electromagnetic rotary machine with windings for controlling the radial position of rotors, which has been proposed by the inventors of the present application. The electromagnetic rotary machine shown in FIG. 2 is disclosed in various documents including (4) "Principles of radial force generation of bearingless motors with a cylindrical rotor operating under no loads" by Akira Chiba, Kouichi Ikeda, Fukuzo Nakamura, Tazumi Deido, Tadashi Fukao, and M. A. Rahman, Electric Society Journal D. Vol. 113, No. 4, pp. 539-547, 1993, and (5) Japanese laid-open patent publication No. 2-193547. As shown in FIG. 2, the electromagnetic rotary machine has two units 16 each connected to a three-phase inverter 17 for controlling currents supplied to the windings for controlling the radial position of rotors, and also to a three-phase inverter 18 for generating a motor torque. Each of the units 16 has four-pole windings for generating a motor torque and two-pole windings for generating radial forces on the rotor. Since each of the units 16 is capable of generating a motor torque and radial forces, the electromagnetic rotary machine has a shorter shaft than general ultra-high-speed motors with magnetic bearings, and can produce a higher output power if its shaft length is the same as those of the general ultra-high-speed motors with magnetic bearings.
The electromagnetic rotary machine proposed by the inventors of the present application has the following features:
(1) The electromagnetic rotary machine, if it has three-phase windings, requires only six wire cables and two three-phase inverters for generating radial forces along two orthogonal axes and a motor torque. PA1 (2) Because the windings for generating the radial forces and the windings for generating the motor torque are separate from each other, the inverter or power amplifier for controlling the radial forces may be of a relatively small power requirement. PA1 (3) Inasmuch as the electromagnetic rotary machine employs the four-pole windings and the two-pole windings, if the rotors are positioned centrally within the stators, there is no mutual coupling, and no induced voltage is developed in the windings for controlling radial forces. PA1 (4) The electromagnetic rotary machine can be used in a wide variety of high-output-power rotary machines which assume a sine-wave distribution of electromotive forces and a sine-wave distribution of magnetic fluxes, including an induction machine, a permanent-magnet synchronous machine, a synchronous reluctance motor, etc.
FIG. 3 of the accompanying drawings illustrates the principles of generation of forces acting radially on a rotor in the electromagnetic rotary machine. As shown in FIG. 3, a stator has four-pole windings N.sub.4 for producing four-pole magnetic fluxes .PSI..sub.4 and two-pole windings N.sub.2 for producing two-pole magnetic fluxes .PSI..sub.2. The four-pole windings N.sub.4 of the stator serve to generate a motor torque on the rotor. If the rotor is positioned centrally in the stator, then when a current flows through the four-pole windings N.sub.4 in a positive direction, the four-pole windings N.sub.4 generate four-pole symmetric magnetic fluxes .PSI..sub.4.
When a two-phase alternating current is supplied to the four-pole windings N.sub.4 and four-pole windings perpendicular thereto, a four-pole revolving magnetic field is generated. The stator may alternatively have three-phase windings. If the rotor has a squirrel-cage type winding, then it generates a torque due to the revolving magnetic field, with the assembly operating as an ordinary squirrel-cage type induction machine. If the rotor has four-pole permanent magnets, then it generates a torque due to the revolving magnetic field, with the assembly operating as an ordinary permanent-magnet motor.
The two-pole windings N.sub.2 of the stator serve to produce forces acting radially on the rotor. When a current flows through the two-pole windings N.sub.2 in a positive direction, they generate two-pole magnetic fluxes .PSI..sub.2 as shown in FIG. 3. Across a gap below the rotor as shown in FIG. 3, the four-pole magnetic fluxes .PSI..sub.4 and the two-pole magnetic fluxes .PSI..sub.2 flow in opposite directions. Therefore, the flux density is relatively high across the gap below the rotor. Across a gap above the rotor as shown in FIG. 3, the four-pole magnetic fluxes .PSI..sub.4 and the two-pole magnetic fluxes 2 flow in the same direction. Consequently, the flux density is relatively high across the gap above the rotor.
When the magnetic fluxes are brought out of equilibrium as shown, the rotor is subjected to radial forces F which are directly upwardly in FIG. 3. The magnitude of the radial forces F can be adjusted by controlling the magnitude of the current flowing through the two-pole windings N.sub.2. To reverse the direction of the radial forces F, the direction of the current flowing through the two-pole windings N.sub.2 may be reversed.
In order to generate radial forces horizontally across the rotor in FIG. 3, two-pole windings may be provided on the stator which are directed perpendicularly to the two-pole windings N.sub.2, and a current flowing through the two-pole windings may be adjusted in magnitude and direction. By thus adjusting the magnitude and direction of the currents flowing through these two-pole windings, it is possible to generate radial forces of desired magnitudes and directions.
In FIG. 3, the four-pole windings N.sub.4 are used to rotate the rotor and the two-pole windings N.sub.2 are used to control the radial position of the rotor. However, it is possible to use the four-pole windings N.sub.4 to control the radial position of the rotor and the two-pole windings N.sub.2 to rotate the rotor.
As far as the inventors know, there has been no report whatsoever on a system for applying such an electromagnetic rotary machine to a large-power-rating variable-speed dynamotor.